Novel light emitting elements including white LEDs and organic electroluminescence elements are now actively developed. In accordance with this, technologies for improving an efficiency of extracting light from a light emitting body are now studied.
FIG. 26 shows a cross-sectional structure of a light emitting device using a general organic electroluminescence element (organic EL element) and how light propagates. On a substrate 101, an electrode 102, a light emitting layer 103, and a transparent electrode 104 are stacked in this order. On the transparent electrode 104, a transparent substrate 105 is provided. By an application of a voltage between the electrode 102 and the transparent electrode 104, light is emitted at a point S in the light emitting layer 103. This light is transmitted through the transparent electrode 104 directly or after being reflected by the electrode 102, and is incident on a point P on a surface of the transparent substrate 105 at angle θ with respect to the surface normal to the surface of the transparent substrate 105. The light is refracted at this point and output to an air layer 106.
Where the refractive index of the transparent substrate 105 is n′1, when the incidence angle θ becomes larger than critical angle θc=sin−1(1/n′1), total reflection occurs. For example, light which is incident on a point Q on the surface of the transparent substrate 105 at an angle of θc or greater is totally reflected and is not output to the air layer 106.
FIGS. 27(a) and (b) show the light extraction efficiency in an assumption that in the above-described light emitting device, the transparent substrate 105 has a multi-layer structure. In FIG. 27(a), where the refractive index of the light emitting layer 103 is n′k, the refractive index of the air layer 106 is n0, the refractive indices of a plurality of transparent layers provided between the light emitting layer 103 and the air layer 106 are n′k−1, n′k−2, . . . , n′1 from the refractive index of the layer closest to the light emitting layer 103, the propagation orientation of the light emitted from the point S in the light emitting layer 103 (angle made with the surface normal to the reflection surfaces) is θ′k, and the refractive indices at the reflection surfaces are θ′k−1, θ′k−2, . . . , θ′1, θ0, the following expression holds based on the Snell's Law.
                                                                                          n                  ′                                ⁢                k                ×                sin                ⁢                                                                  ⁢                                  θ                  ′                                ⁢                k                            =                                                                                          n                      ′                                        ⁢                    k                                    -                                      1                    ×                    sin                    ⁢                                                                                  ⁢                                          θ                      ′                                        ⁢                    k                                    -                  1                                =                …                                                                                        =                                                n                  ′                                ⁢                1                ×                sin                ⁢                                                                  ⁢                                  θ                  ′                                ⁢                1                                                                                        =                              n                ⁢                                                                  ⁢                0                ×                sin                ⁢                                                                  ⁢                θ0                                                                        (                  expression          ⁢                                          ⁢          1                )            
Therefore, the following expression holds.sin θ′k=sin θ0×n0/n′k  (expression 2)
Expression 2 is nothing but the Snell's Law in the case where the light emitting layer 103 is in direct contact with the air layer 106, and indicates that total reflection occurs at θ′k≧θc=sin−1 (n0/n′k) regardless of the refractive indices of the transparent layers provided between the light emitting layer 103 and the air layer 106.
FIG. 27(b) schematically shows a range of light which can be extracted from the light emitting layer 103. The light which can be extracted is contained in a pair of cones 109 and 109′ having the light emitting point S as the apex, twice of the critical angle θc as the vertex angle and z axis along the surface normal to the refractive surface as the central axis. Assuming that the light emitted from the point S radiates in all the orientations at an equal intensity and the transmittance at the reflection surface is 100% when the incidence angle is within the critical angle, the light extraction efficiency η from the light emitting layer 103 is equal to the ratio of the area size obtained by cutting a spherical surface 110 by the cones 109 and 109′ with respect to the surface area size of the spherical surface 110 and is given by the following expression.η=1−cos θc  (expression 3)
In actuality, the transmittance obtained when the incidence angle is within the critical angle is not 100%, and so the light extraction efficiency η is smaller than 1−cos θc. The total efficiency of the light emitting element is a value obtained by multiplying the light emitting efficiency of the light emitting layer by the light extraction efficiency η.
Meanwhile, there are, for example, conventional technologies disclosed by Patent Document Nos. 1 and 2 for improving the light extraction efficiency from a light emitting body. The subject matter disclosed by Patent Document No. 1 is proposed for the purpose of, in an organic EL element, suppressing light which is output from a transparent substrate toward the atmosphere from being totally reflected at a surface of the transparent substrate. The technology is described as being based on the principle that the light extraction efficiency is improved by forming a diffraction grating on an interface, an inner surface or a reflection surface of the substrate and changing the incidence angle of light with respect to the surface from which the light is extracted.
Patent Document No. 2 describes that in order to provide a planar light emitting device providing a high light extraction efficiency in an organic EL element, a plurality of transparent protrusions are formed on a surface of the transparent substrate so that the light can be prevented from being reflected at an interface between the transparent substrate and air.